The invention relates generally to the control of hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO.sub.x) in the exhaust of internal combustion engines. More particularly, the invention relates to the removal of NO.sub.x when the exhaust gases include oxygen substantially in excess of that needed for combustion of the fuel. This is typically the case with lean-burn engines, diesel engines, and other engines currently under development.
In recent years three-way catalysts have been used to remove all of the three principal noxious components in auto exhaust gases. The engines are run with stoichiometric air/fuel ratios and the catalysts are able to remove all three components at the same time, that is, a single catalyst is sufficient over the range of engine operating temperatures. More recently, development of so-called "lean-burn" engines is being driven by the desire to improve fuel economy. However, such engines operate with air-fuel ratios which are far from the typical stoichiometric conditions. Instead of an air-fuel ratio of about 14.55/1 by weight, the lean-burn engine may operate with air-fuel ratios above 18/1, up to about 22-24/1, or even higher ratios of 28-30/1 for advanced lean-burn engines or diesel engines. Under such conditions the engine exhaust will include more hydrocarbons, less carbon monoxide, and less, but still excessive nitrogen oxides. While an oxidation catalyst is capable of removing hydrocarbons and carbon monoxide, since the oxygen content is high, say about 3-10% by volume, it is clear that conditions are not favorable for the reduction of nitrogen oxides. Conventional gasoline engines that operate near the stoichiometric point can experience lean transients where the air-fuel ratio reaches about 15/1. Under these conditions, the chemical equivalence ratio, defined as the sum of oxidizing chemical equivalents divided by the sum of reducing chemical equivalents, of emissions from the engine can reach a value of about 2. (At the stoichiometric point the equivalence ratio is 1.0.) The present inventors are concerned with nitrogen oxide reduction from lean-burn and diesel engines where the chemical equivalence ratio is greater than 3, often greater than 5, and frequently greater than 10.
There are two generally recognized routes to removing nitrogen oxides. First, the nitrogen oxides can be decomposed to the diatomic nitrogen and diatomic oxygen. This reaction is thermodynamically favored, but catalysts which are able to carry out this reaction under the highly oxidizing conditions and high temperatures found in engine exhaust have not yet been found. The second route is the chemical reduction of nitrogen oxides using reducing agents present in the exhaust, such as carbon monoxide, hydrocarbons, and hydrogen. This is considered to be the mechanism of the three-way catalyst. However, such catalysts see the exhaust from an engine operating with a stoichiometric air-fuel ratio which contains little oxygen. When a large excess of oxygen is present, as in a lean-burn or diesel engine, the oxygen, being much greater in volume than the nitrogen oxides would be expected to react with the hydrocarbons, carbon monoxide, and hydrogen, thus removing the reducing agents needed to reduce nitrogen oxides.
In commonly-assigned U.S. Ser. No. 07/990,216 a sequence of catalysts was used to cover the range of operating temperatures experienced with internal combustion engines. The second and third catalysts in the sequence were examples of catalysts of the present invention.